package/libiio: do not force shared object build

libiio correctly builds as a static library, there is no obvious reason
to force the shared object build.

So, let the CMake BUILD_SHARED_LIBS variable handles the type of library
built.

This libiio patch has already been submitted upstream:
  https://github.com/analogdevicesinc/libiio/pull/6



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>